Exploring the dynamic surface features of high and medium entropy oxides

A set of rock-salt oxides was prepared starting from the high entropy oxide Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 O (HEO5) by progressively reducing the number of components, while preserving the native rock-salt structure. The obtained medium entropy oxides were Co 0.2 Ni 0.4 Cu 0.2 Zn 0.2 O (MEO4) and Ni 0.6 Cu 0.2 Zn 0.2 O (MEO3). The performance of the prepared materials towards the selected prototypical probe reaction (CO oxidation) was investigated and correlated with structural and spectroscopic features. The catalytic activity was evaluated over a wide temperature range (50-400 °C). The medium entropy oxide with three cations (MEO3) exhibited the best catalytic performance and became active at low temperature (50 °C). All the materials achieved complete conversion (99.9%) with full selectivity to the desired product (CO 2 selectivity 99.9%) at medium-high temperatures (250 °C < T < 300 °C). We demonstrated that the catalyst surfaces could exhibit different local compositions, with metal enrichment and re-organization induced by the experimental conditions, leading to the formation of novel potentially active surface species. Configurational entropy is a stabilizing term, whose decrease enhances the catalytic activity of the materials, promoting a progressive destabilization of Ni(II) in favor of reduced Ni species on MEO3.